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The origin of new species in evolution


also called phyletic evolution, is the accumulation of changes that gradually transform a given species into a species with different characteristics


also called branching evolution, the splitting of a gene pool into two or more separate pools, which each give rise to one or more new species

biological species concept

Mates of different species are unable to produce vital offspring with each other.

Prezygotic barriers

impede mating between species or hinder the fertilization of ova if members of different species attempt to mate

Postzygotic barriers

prevent the hybrid zygote from developing into a fertile adult even if the sperm cell from one species does overcome prezygotic barriers

Temporal isolation

Species that breed different times of the day, different seasons, or different years cannot mix their genes

Behavioral genes

Courtship rituals that attract mates unique to a species are effective reproductive barriers, even between closely related species.

Mechanical isolation

Morphological differences can prevent successful mating.

Gametic Isolation

sperm of one species may not be able to fertilize the eggs of another species.

Reduced hybrid viability

The genes of different parent species may interact and impair the hybrid's development.

Reduced hybrid fertility

Even if hybrids are vigorous, they may be sterile. If chromosomes of two parent species differ in number or structure meiosis in the hybrids may fail to produce normal gametes.

Hybrid Breakdown

first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of next generation are feeble and sterile.

Morphological species concept

defining species by measureable anatomical criteria

Paleontological species concept

definition of species based on morphological differences known only from the fossil record

allopatric speciation

In _________ ___________, gene flow is interrupted when a population is divided into geographically isolated subpopulations. For ex: a river may change course and split a population of animals that cannot cross it.

sympatric speciation

A mode of speciation occurring as a result of a radical change in the genome of a subpopulation, reproductively isolating the subpopulation from the parent population.


An _____________ is an individual that has more than two chromosome sets, all derived from a single species.
For ex: a failure of cell division can double a cell's chromosome number from the diploid number (2n) to a tetraploid number (4n).


An ___________ is a common type of polyploidy species resulting from two different species interbreeding and combining their chromosomes.

adaptive radiation

the evolution of many diversely adapted species from a common ancestor upon introduction to various new environmental opportunities and challenges

gradualism model

species descend from a common ancestor and gradually diverge more and more in their morphology as they acquire unique adaptions

punctuated equilibrium model

a new species changes most as it buds from a parent species and then changes little for the rest of its existence.

Ecological species concept

Defining species in terms of ecological roles (niches)

Phylogenetic species concept

Defining a species as a set of organisms with a unique genetic tree


Evolutionary change in the timing or rate of an organism's development

Allometric growth

The variation in the relative rates of growth of various parts of the body, which helps shape the organism

Homeotic genes

Any of the genes that control the overall body plan of animals and plants by controlling the developmental fate of groups of cells.


The retention in an adult organism of the juvenile features of its evolutionary ancestors.

Hox gene

provides positional information about how far digits and other bones should extend from the limb.

geographic variation

differences between the gene pools of separate populations or population subgroups.


a graded change in a trait along a geographic axis.


the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

relative fitness

the contribution of one genotype to the next generation compared to that of alternate genotypes for the same locus.

directional selection

shifts the overall makeup of the population by favoring variants at one extreme of the distribution

disruptive selection

favors variants at both ends of the distribution.

stabilizing selection

removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.

heterozygote advantage

individuals who are heterozygous at a particular gene locus have greater fitness than the homozygotes, natural selection will tend to maintain two or more alleles at that locus.

frequency dependent selection

the fitness of any one morph declines if it becomes too common in the population.

neutral variation

A ___________ _________ is one that has no adaptive value one way or the other. Eye color in humans is an example of a ________ _______ normally, you don't live or die or reproduce more or less according to your eye color.

phenotypic polymorphism

existence of two or more distinct morphs (discrete forms), each represented in a population in high enough frequencies to be readily noticeable

genetic polymorphism

The existence of two or more distinct alleles at a given locus in a population's gene pool.

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